1. Field of the Invention
This invention relates generally to electro-mechanical switches, and more particularly to a push button switch having a self-indicating message display.
2. Description of the Prior Art
Push button switches are well known in which a push button projects from the front of a housing and a switch inside the housing is actuated by depressing the push button. The face of the push button can be illuminated from the rear to display a message, legend of the like, by lamps mounted behind the face of the push button. One type of push button switch has a self-indicating message display, in which the face of the push button provides a legend indicating the purpose or use of the switch, and an adjacent message indicating whether the switch is in the "on" or "off" position. This type of push button switch has numerous avionics uses, such as in instrument panels of aircraft. A legend on the face of these push button switches is typically blacked out when the legend is not intended to be read by the pilot. The legend can be backlighted to illuminate it when it is intended to be read. However, in high ambient light conditions, a pilot looking at the face of some push button switches can be read an apparently blacked out legend whether it is illuminated or not. It is desirable to provide a push button switch in which the legend is entirely blacked out when the legend is not intended to be read, so that the legend cannot be read in high ambient light conditions, while ensuring that the legend or a message on the push button is clearly readable, whether in high or low light conditions, when the legend or message is intended to be read.
This invention provides a push button switch having a self-indicating message display in which a legend or message can be entirely blacked out so it cannot be read even in high ambient light conditions.
In some instances, it is necessary to constantly illuminate a message screen on a push button switch to provide a back-lighting for its legend or message. It would be desirable to provide a switch in which the meassage screen is readable in ordinary or high ambient light conditions, without requiring constant electrical power to the lamps to provide back-lighting for the meassage or legend.
If continuous back-lighting is required for a legend on a legend screen, it is desirable that the brightness of the light be sufficient to ensure that the legend can be read in low as well as high ambient light conditions. However, brightly illuminated lamps being on for a long period of time can generate a large amount of heat; and therefore it is desirable to dissipate the heat to avoid an inordinate amount of heat transfer to the face of the push button.
The switch of this invention provides a message screen that is readable in ordinary or high ambient light conditions, without requiring electrical power for back-lighting the message. In instances where continuous back-lighting is required to read the legend on the legend screen, the illumination provided is at a level of brightness sufficient to make the legend readable in low as well as high ambient light conditions, and still the amount of heat generated by the lamp being continuously on is dissipated to avoid an unreasonable amount of heat transfer to the face of the push button.